a) Field of the Invention
The invention relates to an endoscope optical system, in which the object-side part of the objective is a negative lens cluster.
b) Description of the Prior Art
In contrast to photo-objectives and other optical systems, endoscopes have considerable residual defects. On the one hand this is caused by the many components, such as objective, ocular and image dissemination system, and on the other hand by the fact that only simple optical components may be used for reasons relating to cost and manufacture. The adjustment of an endoscope optical system must therefore be optimised by balancing the adjustment of the objective with the adjustment of ocular and image dissemination system.
Image definition and image scale defects occur in the entire optical system. The main image definition defects, such as astigmatism and coma, are corrected in conventional manner in that the so-called rear lenses of the image-side lens cluster are designed as an achromatic objective. Care must now also be taken to ensure that the image scale defects, such as distortion and field curvature, are minimised in the endoscope optical system or completely eliminated. Hitherto the distortion in endoscopes has not been corrected for the reasons relating to manufacture and cost already mentioned. Known endoscopes or endoscope optical systems therefore have a distortion of up to 20%. However, this causes an unreal image, since barrel-shaped distortions usually occur which have to be removed. The same applies to field curvature. As a result of field curvature, the definition planes of the optical system lie at different focus positions for the centre and for the edge.
However, only one focus position may be set for photographic, film and video systems. For this reason the focussing must be mediated between centre and edge of the optical system, that is averaging is required for focussing. However, this has the disadvantage that it results in lower resolution of the optical system. On the other hand an optical system with a flattened image field has greater resolution. The field curvature may be corrected by means of a suitable lens combination. Since it is only reduced by negative refractive powers, adjustment of field curvature is omitted in the ocular and in the image dissemination system in order to keep the components simple with regard to their constructional design and their manufacture. However, adjustment is currently only partially possible in the objective, if for example a negative is used.
The invention proposes an objective with a negative lens cluster for endoscopes or endoscope optical systems, the constructional design of which should exclude the distortions in question. In addition, field curvature should be considerably reduced such that a flattened image field is produced.